The present invention is directed to liquid electrophotographic developer compositions. More specifically, the present invention is directed to a liquid developer with high sensitivity that is suitable for use in electrophotographic devices such as those that perform xeroradiographic medical imaging. One embodiment of the present invention resides in a liquid electrophotographic developer composition comprising an isoparaffinic hydrocarbon, a pigment, a first polymeric resin component comprising a vinyl toluene acrylic terpolymer resin, and a second polymeric resin component comprising an acrylic copolymer resin.
The formation and development of images on the surface of photoconductive materials by electrostatic means is well known. For example, U.S. Pat. No. 2,297,691 discloses an electrophotographic imaging process that entails placing a uniform electrostatic charge on a photoconductive insulating layer, such as a photoconductor or photoreceptor, exposing the photoreceptor to a light and shadow image to dissipate the charge on the areas of the photoreceptor exposed to the light, and developing the resulting electrostatic latent image by depositing on the image a finely divided electroscopic material known as toner. The toner will normally be attracted to those areas of the photoreceptor which retain a charge thereby forming a toner image corresponding to the electrostatic latent image. This developed image may then be transferred to a substrate such as paper and subsequently be permanently affixed to the substrate.
For the electrophotographic process, either dry or liquid developers may be used for development of the electrostatic latent image. Liquid developers generally comprise insulating carrier liquids in which pigments or dyes, resins, charge controllers, and possibly other additives are dispersed or dissolved. In the electric field of the charge image, the charged toner particles are deposited electrophoretically on the charge image.
Various liquid electrophotographic developers are known. For example, U.S. Pat. No. 4,473,629 discloses a liquid developer containing negatively charged toner particles for developing electrostatic charge images comprising a carrier liquid having a high electrical resistivity and a low dielectric constant, such as the aliphatic hydrocarbons commercially available as Isopar.RTM.E, G, H, K, or L, a pigment or dye constituent, a resinous binder, a charge controller, and conventional additives, wherein the pigment or dye constituent is dispersed in a solution of styrene/butadiene copolymer binder in the carrier liquid. This patent also discloses a process for preparing the liquid developer.
Also, U.S. Pat. No. 4,374,918, discloses a thermally stable liquid negative developer comprising an organic liquid carrier such as the aliphatic hydrocarbon Isopar.RTM.G, a pigment, a stabilizing gel on the borderline of solubility in the carrier, a latex which imparts a fixative function to the developer, and a two component charge control agent. According to this patent, the charge control agent consists of a first polymer, soluble in the carrier, having a basic character because of the inclusion of pyrrolidone or hydroxylated alkyl groups; and a second polymer, insoluble or on the borderline of solubility in the carrier, having an acid character because of the inclusion of free halogenated groups, and containing a minor amount of carrier soluble moieties. The two components may constitute separate ingredients, or either or both components may be incorporated into the structure of other developer components.
German Laid-Open Patent Application No. 1,940,985 discloses a liquid electrostatic developer comprising toner particles dispersed in an insulating liquid. The toner particles are aggregates, the outside of which are connected with a binder or fixing resin such as Pliolite.RTM.VT without being completely surrounded or coated by the resin, said aggregates being less than one thousand angstroms.
The liquid suspension developer disclosed in U.S. Pat. No. 4,599,291 comprises an electrically insulating carrier liquid, a pigment, and a block copolymer comprising blocks of vinyl aromatic compounds such as polystyrene and blocks of dienes, such as polybutadiene that have been partially or completely reacted with alkyl mercaptans. The block copolymer is applied to the pigment by melting or precipitation, and metal salts of long-chained organic acids may be present as charge control agents.
Another liquid developer disclosed in U.S. Pat. No. 4,241,159 comprises an insulating carrier liquid with a low dielectric constant, such as the aliphatic hydrocarbons commercially available as Isopar.RTM.G, H, K, and L, in which is dispersed resin-precoated toner particles formed of a pigment or coloring agent precoated with a homopolymer or copolymer of an acrylic or methacrylic acid ester of hydrogenated abietyl alcohol. In addition, the liquid developer may contain other polymeric binding agents to adjust viscosity or to act as charge fixing agents, such as a copolymer of isobutyl methacrylate and stearyl methacrylate comprising about 0.2 percent of methacrylic acid, sold under the trade name Neocryl.RTM.B 702.
Although the above mentioned liquid electrophotographic developers are suitable for their intended purposes, a need continues to exist for high sensitivity electrophotographic liquid developers. A need also exists for liquid developers suitable for use in xeroradiographic medical imaging devices such as the Xerox.RTM.175 System, or those dislcosed in U.S. Pat. No. 4,624,544, the disclosure of which is totally incorporated herein by reference. Also, a need continues to exist for liquid developers that enable formation of medical diagnostic images of high resolution. In addition, a need exists for liquid developers with sufficient sensitivity to enable formation of high resolution medical diagnostic xeroradiographic images while exposing the patient to low doses of radiation. Further, a need exists for high sensitivity liquid developers that are also electrostatically transferrable, such that transfer of the developed image from the photoreceptor to the substrate may be achieved without disruption of the image.